The Human Body

The Cell, the Body's Building Material

Perhaps at some time you have visited a zoo. There you saw huge elephants, tall giraffes, comical little monkeys, strange birds and many other kinds of animals. The animals in the different cages were so different that you must surely believe that they can have little in common.

Yet, all living things actually do have something in common. All living things are made of tiny units, called cells. The huge elephant is made of hundreds of billions of cells, and there are little animals whose whole body is but a single cell. The human body, too, is made of single cells - billions of them.

What do cells look like?

Most cells are so small that you need a powerful magnifying lens to see  one. Some cells are so small that you could put 250 thousand of them on the period at the end of this sentence. Other, however, are large enough to be seen with the unaided eye. Among these large cells are the root hairs of plants, certain seaweeds, and the eggs of animals.

Cells are many shapes. Some are round. Others look like bricks with rounded corners. Still others are long and hairlike. Some cells are shaped like plates, cylinders, ribbons or spiral rods.

What are the parts of a cell?

Looking through a microscope at a single cell from a human body, you can see that the cell is surrounded by membrane. This is the cell membrane. It surrounds the cell in the same way that a balloon surrounds the air within itself.

Within the cell membrane is a material that has a grainy appearance. This material is cytoplasm, which flows about within the cell membrane. Cytoplasm distributes nourishment within the cell and gets rid of the cell's waste products.

Within the cytoplasm is a large dot, This dot is really a sphere and is the cell's nucleus. The nucleus is the most important part of the cell. It directs the cell's living activities. The way in which the cell uses nourishment and oxygen, the way the cytoplasm gets rid of wastes, the way the cell reproduces - all these functions are regulated by the nucleus. If the nucleus is removed, the cell dies.

Of what material are cells made?

The cell membrane, the cytoplasm, and the nucleus of all cells are made of a material called protoplasm. Protoplasm is a living material and makes a living cell "alive". Scientists have analyzed protoplasm into the elements of which it is made. They have found protoplasm to be made of water and many other chemical substances are and how much of each there is in protoplasm, no scientist thus far has been able to put them together properly so as to make living protoplasm is a very complex material.

How are cells organized in a human body?

Cells not only differ in shape, but also in the work each kind of cell performs within a body. A group of cells, all of the same kind, that performs a particular kind of work, is called a tissue. For example, groups of cells that transmit impulses back and forth from the brain to other parts of the body make up nerve tissue. Other kinds of tissue are muscle tissue, connecting tissue, supporting tissue, and epithelial tissue. Epithelial tissue forms the outer layer of the skin, and the surface layer of the cavities in the body, such as the nose, throat, gullet and the stomach. 

When different kinds of tissue are organized to perform a particular kind of work within a body, the tissues form an organ. An eye is an that performs the function of seeing. There are many parts to an eye and each part is made of a particular kid of tissue. When all of the tissues of the eye work together while each tissue performs its separate task, then the eye can perform its function of seeing. Other examples of organs are the heart, liver, tongue and lungs.

Organs of the body are organized into unified systems. Each system performs a particular task for the body. For example, the digestive system, which includes the mouth, teeth, tongue, gullet, stomach, intestines and many glands, performs the function of digesting food.

How is the body like a machine?

Perhaps you have heard an automobile repairman say that a car's ignition system needed fixing. Or maybe it was the cooling system or the brake system, Each one of these systems is made up of several parts, and each system performs a particular task in running the car. All systems must work together if the car is to operate. Do you see the similarity between the automobile's systems and the organ systems of a human body?

the human body is a very wonderful machine. It is more complex, better made and can do more kinds of work under more conditions than any machine that man has so far constructed throughout his history.

Why is the body more useful than a machine?

Man has built giant electronic calculators that can solve mathematical problems in a fraction of the time that a human brain can. Calculating, however, is the work the giant machine can do. It cannot decide what problems it should work on, nor when it should work on them, as the slower but more versatile human brain can. The great calculating machine, with its limited capacity, takes up all the space around the walls of a large room, but the human brain, with its unlimited capacity, can easily fit into a shoe box.

The calculating machine has thousands of parts does not even begin to equal the hundreds of millions of unit cells of which the human body is constructed. If the calculating machine breaks down, it must wait for a repairman to fix it. A break or a cut in some part of the human body can usually be repaired by the body itself


How much skin on a human body?

When you look at a human body, the first thing you see is the skin. The average adult human body is covered with about eighteen square feet of skin. The skin varies in thickness. It is very thin over the eyelids, and quite thick on the palms of the hands and the soles of the feet.  

The skin is composed of two layers. The upper layer is the epidermis. This layer is made of dead, flattened 
cells, which are continually wearing off as we move around. The bottom of the epidermis is made of live cells that die and replace those that wear off on the surface.

Beneath the epidermis is the dermis. This layer of skin is made entirely of living cells. There are many small blood vessels and nerve endings in the dermis. Small coiled tubes in this layer open into the epidermis. These tubes are sweat glands and their openings are called pores. Hairs grow out of the skin and have their roots in the dermis. The openings from which hairs grow are called hair follicles.

What does the skin do?

The skin provides the body with a covering that is airtight, waterproof and, when unbroken, a bar to harmful bacteria. The pigment, or coloring matter, of the skin screens out certain harmful rays of the sun. the skin helps to regulate the temperature of the body. When the body surface is cold, the blood vessels in the skin contract and force blood deeper into the body. This prevents the body from losing much heat by radiation. When the body is too warm, the same blood vessels expand and bring more blood to the surface of the skin. this allows the body to lose heat by radiation. Also, the sweat glands pour out perspiration. The perspiration evaporates, and since evaporation is a cooling process, the skin is further cooled.

How are bones connected?

Every bone in the body - except one meets with another bone. The one bone that does not meet another bone is the U-shaped hyoid bone in the throat. The meeting places of the bones are called joints. There are two kinds of joints: Those about which the adjoining bones do not move, and those about which the bones do move freely. The bones of the cranium are held together by joints of the first kind. These are immovable joints.

What holds the bones together?

Holding these bones together is a kind of a very tough, springy, tissue, called cartilage. Cartilage also joints together the bones of the spinal column. The springy nature of cartilage makes it a good shock absorber. If the lower parts of the spine receive a blow, the cartilage rings that join each vertebra to the one above it, absorb the shock, so that the brain does not feel the blow. If these were not so, every time you took a step, your brain would receive a jolt.

What are ligaments?

The bones at movable joints are held together by thick cords of tough, stringy tissue called ligaments. To aid movement, at least one of the two adjoining bones has a small hollow that contains a lubricating fluid. This fluid helps the bones move smoothly over one another, just as oil helps the parts of an engine move over one another. All the bones of the body and their connecting cartilage and ligaments make up the body's skeletal system. 


What are the muscles?

The bones of the human body have no way of moving themselves. The muscles of the body move the bones and there are more than 600 muscles to move the parts of the skeleton. Muscles make up more than half the weight of the human body. Muscles are made up of bunches of a muscular tissue held tightly together. Muscular tissue is very fibrous, so that a muscle is somewhat like a bunch of rubber bands bound tightly together.

How can you see muscles fibers?

Beef is the muscle of steers. With a pin, pick apart a piece of roast beef. You will easily be able to separate it into long, thin strands that are fibers of muscle tissue. If you have a microscope, place a very thin muscle fiber under a cover-glass upon a glass slide. You will then be able to see that muscle tissue is made up of spindle-shaped cells.

How are muscles attached to bones?

A typical muscle is thick in the middle and tapers gradually toward the ends. It is the ends of a muscle that are attached to bones. One end of a muscle is anchored to a bone that the muscle cannot move. This attachment is called the origin of the muscle. The other end is attached to a bone that the muscle is intended to move. This attachment is called the insertion. For example, the muscle at the front of the upper arm called the biceps - has its origin at the shoulder bone, and its insertion is just below the elbow joint on the bone that is on the thumb side of the forearm. The actual attachment of the end of a muscle to a bone is usually accomplished by a short, tough cord of much the same kind of tissue that makes up ligaments. This connective cord is called a tendon. All the muscles of the body and their tendons make up the muscular system of the body.

What are the two kinds of muscle?

The muscles that move the skeleton are ones that we can move at will. They called voluntary muscles. Among them are the ones that move the eyes, tongue, soft palate and the upper part of the gullet.

There are muscles in the body that we cannot move at will. These are called involuntary muscles is found in the walls of veins and arteries, stomach, intestines, gall bladder, the lower parts of the gullet and in several other internal organs. Thousands of tiny involuntary muscles in the skin move the hair. When you are chilled or frightened and have goose flesh, or goose pimples, the little lumps on you skin are due to the tiny muscles in the skin puling your hairs erect.

What are the differences in muscles?

The eye provides a good distinction between voluntary and involuntary muscles enable you to control the movements of your eye, in order to look in the direction you wish. However, you cannot control at will the muscle that widens and narrows the pupil of your eye. This muscle is involuntary.

But the distinction between voluntary and involuntary muscles does not always hold true. For instance, when you shiver with cold or fright, the muscles that shake your body are voluntary muscles. Ordinarily, you can control these muscles, but, when shivering, you have no control over either starting or stopping the action of these muscles.

They act as if they were involuntary. Certain circus performers can swallow various objects, and then, at will, bring them up without difficulty. These performers have learned to control their involuntary stomach and lower-gullet muscles, as though they were voluntary muscles.

How do muscles move?

Muscle tissue is made of cells whose cytoplasm can contract. When the muscle contracts, it becomes short, and thereby pulls on the bone in which it is inserted. When you want to show someone how strong you are and you "make a muscle" by contracting your biceps, your forearm is pulled up toward your shoulder. If you want to to lower your arm, you relax your biceps and contract triceps, the muscle on the underside of the arm. The contraction of the triceps pulls the forearm straight. You can see that the two muscles of the upper arm work as a team or pair. All the voluntary muscles of the body work in pairs.

How do joints help muscles to move bones?

One way to increase the power used to do work is to apply that power to a lever. A lever is a device that increases work power or range of motion. The joints in the human body act as levers that increase the power of a muscle or increase the distance through which the muscle can move a bone.

If you raise yourself on your toes, you are making use of one kind of lever. The muscles that form the calves of your legs have to do the work of lifting your whole body. You would need to have much larger calf muscles if they had to undergo the strain of lifting your body by a direct pull. Yet you easily raise yourself on your toes, because your foot acts as a lever.

In the act of raising yourself on your toes, your weight bears straight down on the point where your shinbone rests on your ankle bone. The muscles of your calf pull upward on your heel bone, and your foot pivots upward on the fulcrum - the point around which the lever moves - which is formed by the bones that make up the ball of your foot. (Although we say that we raise ourselves on our toes, we actually raise  ourselves on the balls of our feet and steady ourselves with our toes.)

If you reach down and grasp the back of your foot just above your heel, you can feel the strong tendon - called the Achilles tendon - that connects the muscles of your calf to your heel bone. If, now, you raise yourself on the ball of your foot, you can feel the calf muscles tighten and bulge as they contract and pull upward on your heel.


What controls the movements of the body?

Supposed you have dropped your pencil on the floor and want to pick it up. This is the very easy thing to do, something you can accomplish with hardly any thought or difficulty. Yet this simple action causes you to use dozens of your voluntary muscles.

First, you have to locate the pencil. This requires you to move your eyes, and probably also to turn your head, until you have brought the pencil into view. Then you must bend down to reach the pencil, grasp it, and then straighten up again. Not only do dozens of voluntary muscles ring about your motions, but they must do so in just the right order. It would be futile to attempt to grasp the pencil before you bent down to bring your hand within reach. Clearly something is controlling the motions of your muscles. What is it?

The movements of your muscles are controlled by your brain which works through a system of nerves distributed throughout your body. The brain and the nerves, together, make up the body's nervous system. 

What is the cerebrum?

The brain occupies the upper half of the skull. The largest part of the brain, called the cerebrum, consists of two deeply-wrinkled hemispheres of nerve tissue, one hemisphere on each side of the head.

All man's conscious activities are controlled by his cerebrum. It enables him to remember, perceive things, solve problems and understand meanings - in short, to think. Thanks to man's most highly-developed cerebrum, he is the most intelligent of all animals.

What is the cerebellum?

At the back of the skull, and almost covered by the cerebrum, is the cerebellum. This part of the brain, too, consists of two hemispheres. The cerebellum coordinates muscular activity. It is the cerebellum that is responsible for man's ability to learn habits and develop skills. As an infant you learned, after many tries and falls, to stand upright. Learning to walk was another accomplishment that took much time and effort. Now, standing and walking are habits to which you need give no thought, yet both these activities require the use of many muscles in exactly the right order. The cerebellum automatically controls these muscles.

Have you learned to skate or ride a bicycle? At first, you had to think about each move you made, but soon the movements became automatic, so that you did not have to think unless an unusual situation rose. When you were learning, your cerebrum was in control of your movements as you thought about just which muscles you were going to use next. Later, when you knew how to make each movement correctly, your cerebellum took over control from your cerebrum. Although the cerebellum's muscular control is automatic, it is important to remember that the muscles it controls are voluntary muscles.

What is medulla?

The involuntary muscles are controlled by a small part of the brain that is at the top of the spinal cord. This is the medulla. It is a little more than an inch long and is really a thickening of the spinal cord. The medulla controls the beating of the heart, the rate of breathing, the movements of the gullet when swallowing and other vital activities of the body.

What does the spinal cord look like?

The spinal cord extends downward from the medulla through the protecting bony rings of the spinal column. The cord is cylindrical in shape, with an outer covering of supporting cells and blood vessels, and an inner, H-shaped core of nerve fibers. The spinal cord extends through four-fifths the length of the spine, and is a little longer in men than in women, averaging sixteen and one half inches in length. It weighs just about one ounce. Twelve pairs of nerves branch of the spinal cord and pass through the base of the skull into the brain. Thirty-one other pairs branch of the spinal cord throughout its length. This nerve branches

Twelve pairs of nerves branch of the spinal cord and pass through the base of the skull into the brain. Thirty-one other pairs branch of he spinal cord through out ts length. These nerve branches run to all the organs of the body, where they branch again and again, until the smallest branches are nerves which are so thin that they cannot be seen with the unaided eye.

Nerves that extend upward from the spinal cord to the brain pass through the medulla where they cross. Thus, the left side of the brain controls the right side of the body, while the right side of the brain controls the left side of the body.

How is the nervous system like an army telephone network?

An army division is composed of many thousands of men who perform a wide variety of duties. In order to control the activities of so many soldiers, it is necessary to have some system by which the commanding general can learn what is going on in all the units of his division and thus, to give orders to any of these units. In order to accomplish this, a telephone network is set-up.

When a battle is in progress is in progress, soldiers posted near the battle line can telephone reports of action back to their headquarters in order to inform the general of the situation. The general gets the messages from these posts. Using this information, and calling on his training and experience, he issues orders to be followed by soldiers under his command. These orders travel back along the same telephone wires.

Let us follow a similar situation within the human body. Let us suppose that you have accidentally knocked a pencil off your desk and want to pick it up. When the sound of the falling pencil reaches your ears, it causes electrical impulses to move from your ears along two nerves - auditory nerves - and then to your brain. Your ears are similar to the posts near the battle line, your nerves similar to the telephone wires, and electrical impulses similar to the messages that move along the wires.

When the brain receives electrical impulses from the ears, a particular part of the cerebrum perceives the impulses as sound, and passes this information on to another part of the cerebrum, one that is concerned with recognition. This part of the brain calls on the part that stores information the memory. If you have ever before heard a pencil fall, your memory recognizes the sound. Now, you are aware of what has happened.

This situation is similar to that of the general who gets battle reports, and then calls on his training and past experience to help him get a clear picture of what is taking  place at the battle front.

Once your brain is aware of the fallen pencil, it decides to pick up the pencil. Electrical impulses go from your brain to the muscles of your eyes, which then move about seeking to bring the pencil into view. This is similar to that of the general who sends messages to front-line posts asking for more information on the battle.

When the pencil is brought into view, electrical impulses flash back to your cerebrum, which must again go through the processes of perception and recognition, in order to identify the pencil. Here we have new reports coming to back to the commanding general who interprets them.

Having located the pencil, your cerebrum now sends hundreds of electrical impulses along nerves to the many muscles that must be move when you bend over, reach out your arm, close your fingers around the pencil, and then straighten up again. These impulses and the responding muscular movements are similar to messages from the general going out over the telephone wires and the soldiers acting upon the general's orders.

What are nerve cells?

Nerve cells, also called neurons, are specially constructed so as to carry nerve impulses from one part of the body to another. Nerve tissue can conduct extremely small amounts of electricity. Nerve impulses are actually small amounts of electricity.

What are the parts of  a neuron?

Each neuron has a central portion, or cell body, that has a nucleus, cytoplasm and a cell membrane. From one side of the cell body there extend very slender branching threads of protoplasm. These tiny nerve fibers are called dendrites. They look much like twigs at the end of a tree branch. From the other side of the cell body there extends a fairly thick nerve fiber - surrounded by a fatty sheath - which ends in slender, branching threads of protoplasm. These nerve fibers are called axons. Some axons are very short, while others are as much as three feet long. Dendrites conduct nerve impulses to the cell body. Axons carry impulses away from the cell body.

Nerve tissue is made up of a series of neurons arranged so that the branching threads of protoplasm of an axon intermingle with the dendrites of the neighboring neuron. However, the two sets of branches do not actually touch. The gap between the branches is called a synapse. When an impulse moves along a nerve, it must jump across the synapse between one neuron and its neighbor.

What is a reflex action?

If you touch a very hot radiator, you quickly jerk your hand away. You do not think about pulling your hand away - you act automatically. This automatic action is called a reflex action. 

In a reflex action, the nerve impulse takes a special pathway, called a reflex arc. In the case of your touching  the hot radiator, the impulse moved from the skin where it came in contact with the radiator along a sensory nerve to your spinal cord. Here the impulse set off another impulse in a motor nerve running from your spinal cord to your arm muscles. The muscles contracted and pulled your hand away from the radiator. This action took place in about one-tenth of a second. At the same time, the original sensory impulse traveled up your spinal cord to your brain, where you felt it as pain.

How are reflex actions helpful?

Reflex actions are very useful in protecting the body from harm. If you had to think about what movements to make when suddenly threatened with harm, you might become confused and do the wrong thing. The automatic action of your reflexes usually causes you to act correctly and quickly enough to avoid or lessen the danger threatening you. For example, if you suddenly become aware of an object flying through the air toward your face, reflex actions cause you to dodge the object and to close your eyes tightly.

How can you demonstrate a reflex action?

Sit comfortably in a chair, cross your right leg over the upper part of your leg. Feel around just below the kneecap of your right leg for a tendon that runs downward from the kneecap. With the edge of the fingers of your right hand strike this tendon sharply - though not too hard, of course. If you do this correctly, the lower part of your right leg will jump upward, bending from the knee joint. After you have learned to cause this reflex action, wait a few minutes and try it again. This time, you may note that your leg is already in motion before you feel your fingers strike the knee.


What are the sense?

We are made aware of the world around us by means of our senses. For many centuries, man believed that human beings had only five senses: sight, hearing, touch, smell, and taste. Modern scientists have added to the list the senses of pressure, heat, cold and pain. 

There are several steps in the process of sensing. A stimulus acts on the nerves in one of the sense organs. Nerve impulses from the sense organ travel to the brain. In the brain, the impulses are interpreted as a feeling or sensation. For instance, if you stick your finger with a needle, nerve endings in the skin of your finger are stimulated to send impulses to your brain, which interprets the impulses as pain.

It is important to note that, although the brain interprets the impulses as pain, the pain is not felt in the brain, but rather in the finger: that is, the sense organ.

What does the eye looks like?

The organs of sight are the eyes. A human eye is shaped like a ball and is about an inch in diameter. The eye is surrounded by a tough white protective covering. At the front of the eye, there is a transparent circular portion in this covering. Just behind this this transparent portion is a space filled with a clear liquid. At the back of this space is a circular tissue with a hole in it. The tissue is called the iris, and the hole is the pupil. The iris is the colored part of the eye. On the inner edge of the iris, around the pupil, is a ring of a tiny muscles sensitive to light, these muscles contract and narrow the pupil. In dim light, the muscles relax and widen the pupil.

If you stand in front of a mirror in brightly-lit room, you can easily see the pupil of your eye widen and narrow. Cover one eye with your hand for about a minute and a half. Suddenly remove your hand, and look at the eye that was covered. You will see the pupil narrow.

How do we see?

Behind the iris is a transparent circular lens made of tough tissue. Muscles attached to the rim of this lens can focus it upon near or far objects. A beam of light passing through the lens is turned upside down and is reversed from right to left. After passing through the lens light traverses a large spherical cavity that makes up the bulk of the eye. This cavity is filled with a clear liquid through which light passes easily. Around the inner surface of this cavity is a coating of a special nerve endings that are sensitive to light. The sensitive coating is the retina. The nerve endings connect with the optic nerve that leads to the brain.

Light, reflected from an object and entering the eye, is focused by the lens as a reversed image on the retina. The nerve impulses arriving at the brain from the retina are interpreted as an image of the object.

This interpretation also reverses the directions of the image as it was projected on the retina, so that we do not see things upside down and backward.

What does the ear look like?

The ears are the organs of hearing. The part of the ear on the outside of the head helps to a slight extent to direct sound waves into the ear. Sound  waves entering the ear strike the eardrum, or tympanic membrane, and cause it to vibrate. This membrane stretches taughtly across the whole diameter of the ear passage. Touching the inner surface of the eardrum is a tiny bone called the malleus or hammer. The malleus connects by a joint to another little bone, the incus or anvil. And the incus is jointed to a third bone, the stapes or stirrup - so named because it looks like a stirrup. Below and inward from the stirrup are three small cavities filled with liquid that are separated from each other by membranes. The innermost of these membranes connects with nerves that go to the brain.

How do we hear?

When sound waves cause the eardrum to vibrate, the eardrum causes the malleus to vibrate, too. The vibrating malleus strikes against the incus with each vibration. The incus passes the vibration of the stirrup, which, in turn, causes the liquid in the cavities to vibrate. Vibration in the innermost cavity sets up impulses in the nerves that go to the cerebrum. That part of the cerebrum concerned with the sense of hearing interprets the impulses as sound.

This complicated system works remarkably well. It can make you aware of a very wide range and complex combination of sounds, such as those which reach your ear when you are in the presence of an orchestra. Also, your hearing organs can be activated by such small volumes of sound as those which come from a pencil moving over a sheet of paper on the other side of a room from the hearer.

Why do we hear better with two ears than with one?

Blindfold yourself with handkerchief, and sit on a chair place in the middle of a room. Ask someone to move quietly to any part of the room and clap his hands once. Point to where you think he is. Repeat this activity several times as your aide moves quietly from place to place about the room. Have your helper keep score of the number of times you have pointed correctly to the location at which he clapped his hands.

Place a hand tightly over one ear, and repeat the whole experiment. Repeat it a third time, covering the other ear. if your sense of hearing is normal, you will find that your score of correct locations was poorer when you listened with only one ear. From this you can readily understand that using two ears gives you a better perception of sound direction, just as using two eyes gives you a better perception of visual depth.

Why do we smell odors?

The organ of smell is the nose. When taking a breath, you may draw into your nose certain gases intermingled with the gases of which air is made. When the added gases comes into contact with a small patch of epithelial cells on the upper part of the inner surface of your nose, the cells cause impulses to travel along a pair of nerves to your cerebrum, where the impulses are interpreted as odors.

Just how this process takes place is not clearly known. However, since the inside of the nose is always damp, scientists believe that the odorous gases dissolve in the dampness and cause a chemical reaction that stimulates nerve endings in the epithelial cells. This causes the cells to send impulses along the nerves.

Not all gases react with the organ of smell to set up sensations of odor. This is why we call only certain gases - those that do react - odors or smells. The more of an odorous gas that comes into contact with the organ of smell, the stronger is the sensation of odor. This is why we usually draw deep breaths when we sniff about to locate the source of an odor.

Can the sense of smell get "tired" or "lost"?

The sense of smell seems to become fatigued easily; that is, the sensation of odor fades after a short time. Perhaps you have entered a room in which you found a strong odor. After a few minutes,  however, you did not seem to notice the odor at all.

The discharge of mucus that accompanies a severe cold will cause you to lose your sense of smell, because the mucus forms a thick covering over the epithelial cells of the nose and prevents odorous gases from coming in contact with the cells.

The sense of smell is highly developed among a large part of the animal kingdom. This animals use smell as their chief means of learning about their surroundings. In human beings, however, the sense of smell is only mildly developed.

How do we taste things?

Small organs, called taste buds, are located just below the surface of the tongue and in three places in the throat. Certain materials taken into the mouth cause taste buds to produce the sensation of taste. Just how this sensation of taste. Just how this sensation is brought about is not known. Taste like smell, is probably the result of a mild chemical reaction. taste sensations may be divided into sweat, salty. sour and bitter.

Not all tastes are detected by the same taste buds. Those taste buds at the sides and tip of the tongue transmit impulses of saltiness to the brain. The buds at the tip of the tongue detect sweetness, those near the base detect bitterness and those on the sides detect sourness. Thus, there are certain areas of the tongue in which two kinds of taste buds are located: these are the sides and the tip.

The sense of taste is complicated by the fact that one taste may mask or counteract another. For example, the sweetness of sugar will counteract the sourness of lemon juice.

Taste is further complicated by the fact that certain tastes are actually odors. This is true of the taste of an onion. If a bad cold causes you to lose your sense of smell, you will not be able to taste an onion.

How do we feel things?

The chief organs of feeling are free nerve endings in the epithelial cells of the body. On the outside of the body, the skin is the organ of feeling; within the body, it is the epithelial cells that line all cavities, such as the mouth, throat, stomach, intestines, ears, chest and sinuses.

Not all feelings are detected by the same nerve endings. In the skin there are 16,000 that detect heat and cold and more than four million that detect pain. Still others cause the sensation of touch. This latter sensation is in some way heightened by the hairs of the body. If a hairy portion of the body is shaved, its sensitivity to touch is temporarily reduced.

Sensations of feeling within the body are difficult to explain Gas that distends the intestine during an attack of indigestion may cause intense pain. Yet surgeons have found that they can cut, burn, pinch and mash the internal organs of a person without causing the patient any pain.

Are all areas of the skin equally sensitive to the touch?

Blindfold yourself. Ask someone to press lightly the blunt point of a pencil on the upturned palm of your hand. Have him repeat this action, using the points of two pencils held about a quarter of an inch apart. Let your helper continue to do this, alternating irregularly between one and two pencil points. As he does this, try to guess how many points are pressing on your hand each time. You will probably make a fairly good score of correct guesses.

But if you repeated this experiment, using the skin of your upper back, close to your spine, you would not be able to tell wether one or two pencil points were being used. This demonstrates that not all areas of the skin are equally sensitive to touch.


How does the body use food?

We have learned that the blood carries nourishment to the cells of the tissues. The nourishment comes from the food we eat. Certainly, food in the form in which we put it into our mouths could not be carried by the blood. Before food is in a for that enables it to nourish the tissues, it must be greatly changed. This process of change is called digestion. 

The mouth, esophagus or (gullet), stomach, small intestine and large intestine form a continuous tube about thirty feet long called the alimentary canal. Food passes through the alimentary canal during the process of digestion. The liver and the pancreas, two large glands, are also important in the digestion of food. The alimentary canal and these two glands make up the body's digestive system.

How does digestion begin in the mouth?

One of the constituents of food is starch. When food that contains starch is chewed, the saliva in the mouth brings about a chemical change in the starch. As a result of this change, the starch becomes a kind of sugar that is easy for the body to use as nourishment for the cells.

A substance, such as saliva, that changes food into a form that can be used by the body is called an enzyme. Enzymes are secreted by glands. Saliva is secreted by saliva glands in the roof and floor of the mouth.

Only starch can be digested in the mouth. Fats and proteins, the two other main constituents of food., must be digested farther along in the alimentary canal.

How do teeth aid digestion?

Since food, whether digested in the mouth or other part of the alimentary canal, must be swallowed, the food must first be broken up into small pieces. As we chew, our teeth cut and grind food into small pieces that are wetted by saliva, and finally formed by the tongue into lumps that we can easily swallow.

A tooth is a remarkable structure. The part of the tooth above the gum is the crown; below the crown, and covered by the gum, is the neck; below the neck is the root that lies in the socket of the jaw bone. A tooth has an outside covering of enamel, the hardest material in the body. Inside the enamel, and forming the main part of the tooth, is dentine. It looks like bone but is harder. In a cavity in the center of the tooth is pulp, which contains blood vessels and nerves.

What are carbohydrates?

When you look on the shelves of a supermarket, you see such a variety of food that is hard to believe all the different kinds can be divided into a few food elements. But this is true.

One food element is called carbohydrate. Carbohydrates are made up of the chemical elements carbon, hydrogen and oxygen. Starches and sugars, bread and macaroni and rock candy, too, are some carbohydrates. The human body uses carbohydrates as a source of energy. If the body has more carbohydrates that it can use, it may change them into fat, which it stores.

What is fat?

Another food element is fat, which is a better source of energy than carbohydrate. Butter, margarine, lard and olive oil are a few examples of fat as well as the white irregular streaks in a beef-steak and around the edges of the streak.

What is protein?

The third main food elements is protein which is manufactured in the bodies of green plants. When human beings or cattle eat green plants, the plant protein is changed into muscle. When human beings eat meat, which is cattle muscle, they make use of their best source of protein. Meat, also, builds muscle in the human body.

What are vitamins?

Many foods contain small amounts of substances called vitamins, which are necessary to the health of the body. Vitamins are named by means of the letters A, B, C, D, and K.

Vitamin A is important for healthy eyes, skin, mucous membranes and for normal growth. Vitamin B is needed for good appetite, good digestion of carbohydrates, normal growth and health of nerves and muscles. Vitamin C is important for growth, the development of teeth, good skin and healing. Vitamin D is needed for strong bones and teeth. Vitamin K is important for the clotting of blood and normal liver function.

Even if we eat enough food, we will not be healthy unless the food contains sufficient vitamins.

What are minerals?

Other food elements are called minerals. These are small amounts of certain chemical elements. For example, the elements phosphorous and calcium are needed for healthy teeth and bones.

In order to be healthy, we must give our bodies proper amounts of these food elements. How are we to know just what foods will provide the right amounts? Scientists have worked out the answers, and when our diet includes the proper amounts of each food element, we are then said to be eating a balanced diet. 

What is the process of digestion?

A chicken sandwich, for example, contains starch fat and protein. The bread is mainly starch, butter is fat and chicken is protein.

When a piece of the sandwich is chewed, the starch is being digested by saliva. When mouthful of the sandwich is swallowed, it passes into the esophagus. This is the muscular tube that contracts along its length to push the food down into the stomach. In the stomach, which is a muscle, the food is churned about while digestive juices pour in from glands in the stomach wall. Eventually, the churning action moves food out of the stomach and into the small intestine.

What does the small intestine do?

The greater part of the digestive process takes place in the small intestine. Here the protein and the fat are finally changed into forms that can be used by the tissues. The liver contributes to this digestive process by secreting into the small intestine liquid called bile. the pancreas secrets pancreatic juice which further aids in dissolving foods.

The small intestine undergoes continual muscular contraction called peristalsis. This action pushes the digested food into the large intestine. The surface of the small intestine has a large number of threadlike projections called villi. the digested, liquefied food is absorbed through the villi. now, the food is in the bloodstream. As we have learned, the blood carries the food to the cells in the tissues, which use the food to provide the body with energy and material for repair. Not all the parts of the chicken sandwich can be digested. Those parts which are indigestible pass through the large intestine to its lower part, called the rectum. Eventually, the indigestible food is eliminated from the rectum through the anus.


Although the study of anatomy is more than 2,000 years old, it was not until the English physician William Harvey described the circulation of the blood, at the beginning of the 17th century, that men knew what work the heart did in the body. the heart had been carefully dissected and described, yet no one knew its use. The heart is a very efficient pump that moves blood through the body. The heart is a muscle that contracts and relaxes abut seventy times a minute, for all the minutes of all the years of your life. Each contraction and relaxation of the heart muscle is a heartbeat. You have more than 100,000 heartbeats everyday. Each heartbeat pumps about two ounces of blood. This results in about 13,000 quarts of blood being pumped each day.

What does the heart look like?

The heart is divided into four chambers. The upper two chambers are called auricles; the lower two are called auricles; the lower two are  called ventricles.  Each auricle is connected with the ventricle below it by a valve that allows  blood to flow from the auricle to the ventricle, but not in the opposite direction. The heart also contains a network of nerves that naturally regulates the pumping operation. One of the wonders of modern surgery is the heart transplant, first demonstrated to be operationally feasible in humans by Dr. Christian Barnard.  By this means, the deteriorated heart of a patient who would otherwise not have long to live may be physically replaced by the healthy heart of a person who has just died. Several people have been able to to resume normal lives, thanks this medical miracle.

How can you hear a heartbeat?

Obtain two small funnels and a length of a rubber tubing about one or two feet long. Into each end of the tube, place the snout of one of the funnels. Now, place the rim of one funnel on the chest of a friend, and place the other funnel to your ear. The "lub-dupp, lub-dupp" you hear is and closing. A doctor listens to the heartbeat by using a stethoscope.

What work does the blood do?

Blood has been called "the river of life." This is an appropriate description, because the blood supplies the cells of the body with the materials they need for nourishment and repair, and it removes wastes from the cells. In addition, the blood contains cells that fight disease and substances that repair cut or bruised parts of the body. The blood is made of both liquid and solid parts. The liquid is called plasma. The solid parts are red corpuscles, white corpuscles and platelets. The word corpuscles is the Latin word for "little body."

What are red corpuscles?

More than nine-tenths of the blood consists of red corpuscles. They are so small that a large drop of blood contains more than 250 million of them. They are disc-shaped and concave on each side. These corpuscles contain a substance called hemoglobin, which is a compound of iron. Hemoglobin can combine very well with oxygen from the air in the lungs. It is the task of the red corpuscles to carry oxygen to cells in all parts of the body, and upon reaching these cells, to give up the oxygen to them.

When hemoglobin combines with oxygen, it turns bright red. That is why blood running out of a cut is always red - the hemoglobin is combining with the oxygen of the air. Red corpuscles live only about fifty to seventy days, and thus, they must be replaced continuously. we learned that the interior of a bone contains reddish tissue, which is due to the presence of red blood cells. Within the marrow of some bones, red cells are formed. if a person lacks sufficient red corpuscles, he is said to have the disease anemia. He is usually listless and thin, because his cells do not receive enough oxygen. Some types of anemia may be cured by adding sufficient iron to an anemic person's diet.

How does the blood fight disease?

Most white corpuscles are larger than red ones, and there are fewer white corpuscles in the blood than red ones. For approximately every 800 red cells there is only one white cells. White corpuscles have no definite shape, and move about by changing their shape. Disease is caused by an overabundance of harmful bacteria within the body, and it is the function of the white corpuscles to destroy bacteria. To destroy a bacterium, a white cell moves over to the bacterium and then engulfs it. Once the bacterium is inside the white cell, it is digested. When large numbers of harmful bacteria invade the blood, the body automatically increases the number of white corpuscles produced by the bone marrow. Then the body has sufficient white cells to destroy most of the invading bacteria.

How does blood clot? 

You know that when you cut yourself, the blood flows out of the wound for only a short time. Then the cut fills with a reddish solid material. This solid is called  blood clot. if blood did not clot, anyone with even a slight wound would bleed profusely. Indeed, the blood of certain persons does not clot, a condition known as hemophilia. The platelets are the particles in the blood responsible for causing it to clot. When blood flows from a cut, it  carries platelets. When air comes into contact with the platelets, the oxygen in the air causes the platelets to disintegrate and release a substance that combines with certain substances in the plasma. This combination forms a substance called fibrin. Fibrin is in the form of a network of tiny threadlike fibers that trap the cells of the blood to form a dam which holds back the further flow of blood.

How does blood move through the body?  

Since the heart pumps so much blood, it must be clear that the same blood must pass through the heart many times in the course of a day. This is true, for the round trip of blood from the heart to distant parts of the body and back takes less than a minute. The round trip to nearer parts of the body takes an even shorter time. The blood takes two main paths in its trip through the body. When the right ventricle of the heart contracts, blood is forced into a large artery that leads to the lungs. (An artery is an elastic tube that carries blood away from the heart.) Here the red cells of the blood take up oxygen from the air in the lungs They also give up carbon dioxide. From the lungs, the blood flows through the veins that lead back to the heart. (A vein is an elastic tube that carries blood toward the heart.) The blood enters the left auricle and passes through the valve leading to the left ventricle. When the left ventricle contracts, the blood flows into another large artery. This artery branches into small arteries that branch several times more into smaller and smaller arteries. The smallest arteries are in the tissues, and are called capillary arteries. From the capillaries, the blood transfers nourishment and oxygen to the cells and removes carbon dioxide and other wastes.

Capillary arteries connect with capillary veins. These tiny veins connect with larger and larger veins as they approach nearer to the heart. Blood flowing through the veins eventually reaches a large vein that enters the right auricle of the heart. From the right auricle, the blood flows through the valve leading to the right ventricle, and thus it ends a complete round trip through the body. The heart, the blood and the veins and arteries make up the body's circulatory system.


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